soc/intel/quark: Read the rmu.bin file from read-only region

Always read the rmu.bin file from the read-only section of the SPI
flash.  Without this change vboot attempts to read this file from the
A or B section of the flash.

TEST=Build and run on Galileo Gen2

Change-Id: Ied8eaa2cd37645bf401aa957936943946bfd6182
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/18803
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/intel/quark/include/soc/romstage.h

@@ -32,6 +32,7 @@
 asmlinkage void *car_stage_c_entry(void);
 void clear_smi_and_wake_events(void);
 void disable_rom_shadow(void);
+void *locate_rmu_file(size_t *rmu_file_len);
 void report_platform_info(void);
 int set_base_address_and_enable_uart(u8 bus, u8 dev, u8 func, u32 mmio_base);
 void pcie_init(void);